I was a lighting designer for a number of years. The pros who shot my shows seemed to pretty consistently always carry the same 2 lenses; 50 f1.4 and 70-200 f2.8 both Nikon. For my own documentation I used a 50 1.4 exclusively.

The Tamron is less expensive and has better IQ than the Sigma so I think it makes slightly more sense. However, I forgot about the motor thing - I don't know if the Tamron has a focus motor or not so that may be the killer right there. That would have to be part of the research.

The Tamron 70-200mm f/2.8 has a focus motor in Nikon mount.

That seems to be the trend with new Tamron lenses in Nikon mount, probably to allow them to be used on Nikon bodies without a focus motor built in (D40, D40x, D60, D5000).

The downside to that approach is that you don't improve AF speed as much if you upgrade to a camera with a faster motor in the camera body, since you're going to be limited by the slower motor Tamron is using in their lenses (as they don't have any lenses with faster Ultrasonic type motors yet).

1st post here, and this is exactly the question that I asked myself the other night when trying to shoot my 5 year old daughter on the stage. Nice to see so many responses, and such enthusiasm to help as well.